[imap] ignore extra items from imap.fetch()

`imap.fetch(id_set, "RFC822")` may return multiple items per
message where only one has the full email under the "RFC822"
map key.

Ignoring items without the "RFC822" is a simple workaround.

I am seeing an item with "RFC822" and a second item with "FLAGS".
When Mail.read_from_string() runs on the FLAGS item, an empty
message without a date is parsed and `mail.date.to_time.gmtime`
fails on the second message:

    A plugin had an unrecoverable error. Will restart this plugin.
      Plugin: <LogStash::Inputs::IMAP type=>"imap", host=>"mail.example.com", user=>"logstash@example.com">
      Error: undefined method `to_time' for nil:NilClass
      Exception: NoMethodError
      Stack: lib/logstash/inputs/imap.rb:89:in `parse_mail'
    lib/logstash/inputs/imap.rb:64:in `block (2 levels) in check_mail'
    lib/logstash/inputs/imap.rb:62:in `each'
    lib/logstash/inputs/imap.rb:62:in `block in check_mail'
    lib/logstash/inputs/imap.rb:60:in `each_slice'
    lib/logstash/inputs/imap.rb:60:in `check_mail'
    lib/logstash/inputs/imap.rb:49:in `block in run'
    vendor/bundle/ruby/1.9/gems/stud-0.0.17/lib/stud/interval.rb:11:in `call'
    vendor/bundle/ruby/1.9/gems/stud-0.0.17/lib/stud/interval.rb:11:in `interval'
    lib/logstash/inputs/imap.rb:48:in `run'
    lib/logstash/pipeline.rb:151:in `inputworker'
    lib/logstash/pipeline.rb:145:in `block in start_input' {:level=>:error, :file=>"logstash/pipeline.rb", :line=>"156"}

That's during testing against old Courier IMAP versions on a
Debian box:

    $ dpkg -l courier-base courier-imap-ssl
    ||/ Name           Version        Description
    +++-==============-==============-============================================
    ii  courier-base   0.65.0-3       Courier mail server - base system
    ii  courier-imap-s 4.8.0-3        Courier mail server - IMAP over SSL